Field of the Invention
The present invention relates to a method for synchronization of a first clock signal, which can be generated in a data receiving station, with a second clock signal which is used in a data transmitting station. The synchronization is in each case carried out by evaluating a value and a time of timemark data which represent counts that are dependent on the clock signal frequency, which timemark data are transmitted to the data receiving station from the data transmitting station.
Methods of this type are mainly used in digital transmission systems. A practical example will be explained in the following text on the basis of a system operating in accordance with the MPEG-2 Standard.
The MPEG-2 Standard is a compression and transmission standard which is configured in particular for digital video data and which is expected to be or become the most important standard of its type in the coming years. It is already used to a considerable extent in digital television sets, video recorders, etc..
In a system operating in accordance with the MPEG-2 Standard, data are intended to be transmitted from a data transmitting station to a data receiving station. In the data receiving station, the data will be further processed, then a clock signal which is synchronized to a clock signal used in a data transmitting station has to be generated in the data receiving station.
In contrast to analog video systems, where it has been possible to synchronize the data receiving station and the data transmitting station using TV synchronization signals transmitted together with the video data, digital systems require a different procedure, owing to the fact that it is not necessary to transmit the known TV synchronization signals.
In the systems under consideration here, that is to say systems which operate in accordance with the MPEG-2 Standard, the different procedure consists of the data transmitting station transmitting specific timemark data, or so-called time stamps, to the data receiving station from time to time.
The configuration of the data transmitting station and of the data receiving station which needs to be provided for synchronization using time stamps, as well as the generation and the evaluation of the time stamps in the data transmitting station and the data receiving station, respectively, are described in the following text with reference to a practical example.
The clock signal which is intended to be used for synchronization, that is to say the clock signal used by the data transmitting station (for example a coder) is assumed to be a clock signal whose frequency may be 27 MHz. The permissible error from the nominal frequency is assumed to be .+-.810 Hz, with the maximum permissible drift rate being defined as 0.075 Hz/s. The values thus exactly match the specifications provided by the MPEG-2 Standard.
The clock signal from the data transmitting station is used to trigger a counter which is provided there (in this case a 42-bit counter), in which case the count of the counter is incremented by 1 per clock cycle. The respective current count of the counter is transmitted to the data receiving station (for example a decoder) at specific time intervals. The data, which represent the count that is dependent on the clock signal frequency, are the timemark data or time stamps already mentioned above.
The time intervals at which such time stamps are transmitted differ. In the MPEG-2 Standard, they are a maximum of 100 ms (for the so-called transport stream) or a maximum of 700 ms (for the so-called program stream).
The data receiving station receives the time stamps, in which case the time of reception is in each case that time at which the last bit of the respectively transmitted time stamp is received.
In the data receiving station, the counter count which is represented by a respective time stamp from the data transmitting station is compared with the count which a counter provided in the data receiving station has reached at the time when the time stamp is received.
The counter in the data receiving station counts as a function of the clock signal (for synchronization) generated there. In other words, its count is incremented by 1 per clock cycle of the clock signal generated in the data receiving station.
If and for as long as the comparison of the counts indicates that they are the same or have a constant difference, it may be assumed that the clock signal of the data receiving station and the clock signal of the data transmitting station are at the same frequency and are synchronized. If not, that is to say if the count difference varies and thus indicates inaccurate or faulty synchronization, a clock signal generator which produces the clock signal to be synchronized in the data receiving station is readjusted in order to achieve synchronization again as quickly as possible.
The knowledge and following of the difference between the counts is in general important not just for clock signal synchronization, since the count of the counter in the data receiving station may also be used, inter alia, to set defined reference times and to determine output or passing-on times, which are predetermined by the data transmitting station and are related to the reference times, at which useful data (video and/or audio data) transmitted to the data receiving station must be output or passed on.
If the continual comparisons of the counts to be compared with one another indicate differences which fluctuate frequently and/or considerably, then, in order to achieve more accurate synchronization, the time intervals between the clock signal generator readjustments can be shortened by outputting and evaluating the time stamps more frequently. The maximum possible synchronization errors can be considerably reduced in this way.
However, as is known, the synchronization is also influenced by propagation time fluctuations and by so-called jitter phenomena in the course of data transmission from the data transmitting station to the data receiving station, to be precise, in particular, if this affects the time stamp transmission.
The propagation time fluctuations and jitter phenomena are randomly varying fluctuations of the signal propagation time. Their magnitude depends, inter alia, on the transmission route (satellite, cable, ATM network etc.), and may vary between a few nanoseconds and several milliseconds.
The main irregularities which can be observed when jitter phenomena occur are shown in FIG. 7.
FIG. 7 shows the association between the time of transmission and the time of reception of time stamps which are sent from the data transmitting station to the data receiving station.
In the ideal case, that is to say when the transmission route characteristics affecting the propagation time of the time stamps are constant over time, there is a linear relationship between the respective time stamp times of transmission and the associated times of reception. The ideal case is illustrated in FIG. 7 by a straight line marked I.
However, since the relationships are not ideal, discrepancies occur from the linear relationship represented by the straight line I. In other words, the respective time stamps represented by dots in FIG. 7 are received by the data receiving device at times which differ from the expected (defined by the straight line I in FIG. 7) times of reception or nominal times of reception. The actual times of reception are, in most cases, before or after the respective nominal times of reception, in which case the respective intervals between the actual times of reception and the nominal times of reception may vary randomly in terms of mathematical sign and magnitude.
These jitter phenomena influence the desired synchronization, to be precise owing to the change this causes in the choice of the count values to be compared with one another.
Specifically, the count which is to be compared with the count represented by the received time stamp is actually that counter count of the data receiving station which the counter is at, at the time when the time stamp is received. Thus, not only the time of reception of the respective time stamps, but also the count to be compared with this, fluctuates with the fluctuation in the transmission time of the time stamps from the data transmitting station to the data receiving station.
Overall, this may lead to the jitter phenomena causing the clock signal generator that generates the clock signal to be synchronized to be readjusted, resulting in incorrect synchronization, or at least having an influence on the readjustment, preventing exact synchronization. This, together with the fact that sudden changes resulting from this may result in the frequency of the clock signal generated in the receiving station are, of course, a problem which must be overcome.
This can be accomplished, for example, by providing a low-path filter which subjects those control signals which are intended for triggering of the clock signal generator in the data receiving station to low-pass filtering before they are applied to the clock signal generator.
The clock signal generator in this case is assumed to be a voltage-controlled crystal oscillator (VCXO), whose output signal is of a frequency which depends on a control voltage CV which is input.
The voltage-controlled crystal oscillator furthermore includes a counter crystal oscillator VCXO. The count of the counter CNT is in this case incremented by 1 per clock cycle of the generated clock signal.
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The count of the counter CNT is fed to a subtraction element, to which the counts which are represented by the respective time stamps are also fed. In the subtraction element, the difference between those counts that are associated with one another by definition is formed in each case after a time stamp has been received.
The output signal from the subtraction element is subjected to low-pass filtering in a low-pass filter, and is passed from there, as the control voltage CV, to the clock signal generator. The low-pass filtering is used to prevent suddenly changing triggering of the crystal oscillator VCXO, which is possible particularly when jitter phenomena occur. The low-pass filter smoothes the difference signal obtained from the differentiation unit, in other words its time waveform, and in this way ensures that the interfering influence of the jitter phenomena is kept within limits.
However, on the other hand, the provision of the low-pass filter has the disadvantage that changes in the frequency of the clock signal of the data transmitting station will, under some circumstances, have only a gradual effect, or no effect whatsoever, on the frequency of the clock signal to be synchronized of the data receiving station. Furthermore, a relatively long time passes after the data receiving station has been switched on or reset before it is running in synchronism with the data transmitting station.